This invention relates to a passivant system for semiconductor junctions, and more particularly, to a multi-layer passivant system.
A recently utilized passivant system for high voltage semiconductor junctions includes a layer of semi-insulating material, most usually oxygen doped polycrystalline silicon, applied either directly on the semiconductor surface or over an oxide layer on such surface. Over this semi-insulating layer there is provided an oxide layer, usually silicon dioxide (SiO.sub.2), over the oxide layer there is provided a relatively thick glass layer and over the glass layer there is provided yet another oxide layer.
With the above described systems, fringing fields can be developed from stray charge in the encapsulant material around the device and can cause a build up of charge at the interface between the semi-insulating material and the semiconductor surface. Such charge build up alters the depletion region when the device is reverse biased and can result in degradation of the breakdown voltage of the device or an increase in its leakage current. Moreover, these fringing fields may also cause the chemical degradation of the encapsulant material. In order to minimize the presence of stray charge and its adverse consequences, a high degree of cleanliness or special materials are used during the passivation, metallization, encapsulation and packaging processes used in making semiconductor devices. This results in a relatively high cost of manufacture.
In addition, fringing fields developed during reverse bias of the device extend from the semi-insulating materials into the encapsulant material around the device. These fields cause the migration of impurity ions in the encapsulant toward the dielectric and also attract external impurity ions to the encapsulant. If the encapsulant is a glass or resin material, it deteriorates and absorbs additional impurities and moisture which impurities ionize in the presence of the moisture. These additional impurity ions also migrate to the dielectric. Eventually, these ions induce a large current flow in the device which because of the high voltage during device operation generates heat which can cause catastrophic failure of the device.